This invention relates to communication systems and more particularly those systems where digital frequency synthesis is used for remote communication applications.
Frequency synthesis is used to track the frequency of an incoming data stream. Such synthesis typically uses some means to lock on to the frequency of the incoming data stream. One such means is the well known phase lock loop (PLL) a block diagram for which is shown in FIG. 1.
As is shown in FIG. 1, frequency synthesizer 10 includes a divide by n circuit 12 in the form of a digital counter which produces an output cycle for each n input cycles. Thus the signal at the output of circuit 12 has a frequency of fi/n, where fi is the frequency of the signal at the input to synthesizer 10.
Phase detector 14 receives the output of circuit 12 and also receives an input from loop 16 comprising loop filter 16a, voltage controlled oscillator (VCO) 16b and divide by m circuit 16c which is also in the form of a digital counter that produces an output cycle for each m input cycles. The output of the PLL is at the output of the VCO 16b and if that output has a frequency of fo then the signal produced at the output of divide by m circuit 16c has a frequency of fo/m. Thus the input frequencies to phase detector 14 are fi/n and f0/m.
Under locked steady state conditions the PLL causes the input frequencies to phase detector 14 to be exactly equal, that is, fi/n=fo/m. The output frequency can then be expressed as:
fo=(m/n)* fi.
One of the drawbacks to this method of frequency synthesis is that the PLL needs time to lock on to the frequency. During this time the data can not be reliably retrieved from the data stream necessitating a sacrificial preamble in front of every transmission.
An alternative technique to frequency synthesis in overcoming communication issues is commonly referred to as Store-Forward. One such prior art implementation for the Store-Forward technique is circuit 18 shown in FIG. 2. The Store-Forward method samples the incoming data stream 20 at sampler 28 with either a clock 30 supplied with that stream or with a clock recovered from the data stream by a clock recovery circuit 22 which is shown in dashed lines in FIG. 2. After a completed data frame has been received it is stored in a buffer 24 until it is retransmitted. Circuit 18 also includes a serial to parallel converter 32, a parallel to serial converter 34, an oscillator 26 and control logic 36.
The Store-Forward method requires that enough data be stored to compensate for any differences between the oscillator 26 and the oscillator of the device that originated the message. The major deficiency of this approach is that a significant time delay is introduced into the communication link and therefore can reduce the overall system bandwidth.
The invention is a method for tracking the frequency of a remote oscillator that has been used to generate a received message data stream. The method synchronizes the received message data stream to the frequency of a voltage controlled oscillator. The method also uses the synchronized received message data stream to generate a first time reference to sample the synchronized received message data stream into a sampler. The method further uses the voltage controlled oscillator frequency to generate a second time reference to read the synchronized received message data stream out of the sampler. The method also further uses the first and second time references to determine how much time has accumulated between the voltage controlled oscillator frequency and the remote oscillator frequency. The method further samples the accumulating time difference at a predetermined rate; and adjusts the voltage controlled oscillator frequency in steps each of a predetermined amount to substantially nullify the accumulating time difference.
The invention is also a method for tracking the frequency of a remote oscillator that has been used to generate a received message data stream. The method synchronizes the received message data stream to the frequency of a voltage controlled oscillator. The method also determines an accumulating difference in time between the voltage controlled oscillator Frequency and the remote oscillator frequency using both a first time reference that is generated from the synchronized received message data stream and a second time reference that is generated from the voltage controlled oscillator frequency. The method further samples the accumulating time difference at a predetermined rate. The method also further adjusts the voltage controlled oscillator frequency in steps each of a predetermined amount to substantially nullify the accumulating time difference.
The invention is further a method for tracking the frequency of a remote oscillator that has been used to generate a received message data stream. The method synchronizes the received message data stream to the frequency of a voltage controlled oscillator. The method also generates a first time reference from the synchronized received message data scream. The method further generates a second time reference from the voltage controlled oscillator frequency. The method also further determines an accumulating difference in tame between the voltage controlled oscillator frequency and the remote oscillator frequency using the first and second time references. The method further samples the accumulating time difference at a predetermined rate; and adjusts the voltage controlled oscillator frequency in steps each of a predetermined amount to substantially nullify the accumulating time difference.
An apparatus for tracking the frequency of a remote oscillator that has been used to generate a received message data stream. The apparatus has a voltage controlled oscillator having an adjustable frequency and a sampler. The apparatus also has a first circuit for synchronizing the received message data stream to the adjustable frequency of the voltage controlled oscillator and for generating from the synchronized received message data stream a first time reference to sample the synchronized received message data stream into the sampler and for generating from the voltage controlled oscillator frequency a second time reference to read the synchronized received message data stream out of the sampler. The apparatus further has a frequency discriminator responsive to the first and second time references to determine an accumulating difference in time between the voltage controlled oscillator adjustable frequency and the remote oscillator frequency, the frequency discriminator sampling the accumulating time difference at a predetermined rate; and a control loop to adjust the voltage controlled oscillator adjustable frequency in steps each of a predetermined amount to substantially nullify the accumulating time difference.